fix: handle NaN scores in sort_by comparison

[echobt]

Description

This PR fixes a bug where scores (potentially from embedding engine or numerical issues) would cause non-deterministic sorting behavior in search results due to the use of with .

Changes

• Updated logic in to explicitly handle values by treating them as smaller than any other number (pushing them to the end of the sorted list).
• Applied the same fix to where similarity scores are sorted.

Verification

• Created a reproduction test case demonstrating that the old sort was unstable with NaNs.
• Verified that the new sort correctly orders real numbers descending and pushes NaNs to the end.
• Ran existing test suite to ensure no regressions.